So in summary, the difference is that inline images are weblinks to a URL that you must guarantee on which you have arranged to provide hosting.
Images attached under "Additional Options..." are loaded to the Arduino server which continues to host them - presumably for the life of the forum.
Now if you are really sneaky, I fancy you can attach the image, thus loading it to the Arduino server, then use the attached link to provide an inline image, and in modifying your post, remove the attachment at the bottom. Or not.